1. Field of the Invention
The invention concerns joining printed sheets to form a pad, in particular to manufacture calendars from pre-printed month sheets.
2. Description of the Prior Art
There is known from French Pat. No. 1 604 324 and its patents of addition 2 075 854 and 2 082 540 a method of joining printed sheets without using glue by reactivating resins included in the ink by heating and pressing the sheets. According to these documents, the assembled sheets are stacked between the plates of a heated press under a low pressure of 2 to 4 kg/cm.sup.2, as stated in those documents, that is 2 to 4 daN/cm.sup.2.
However, the slowness of the method generally makes it impossible to integrate it in-line with the preceding operations (assembly of the printed sheets) or the subsequent operations, unless these operations are slowed down to an exaggerated degree, which is not highly cost-effective.
Also, not all kinds of paper have been suitable for use of this method, in particular porous paper.
Also, because of how they dry, either by evaporation of solvents (helio (photogravure) inks) or by acceleration of the process by infra-red or ultra-violet light, some inks lose the capability of reactivating their adhesive properties and so their suitability for use in this method.
An object of the invention is to eliminate the foregoing disadvantage by providing improved adhesion with a pressing time in the order of one second.